This invention relates to fuel cells in general and a method of managing the performance a fuel cell in particular.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OHxe2x88x92) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. Similarly, at the fuel cell cathode the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the metallic external circuit. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically extracted as vapor. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat at the temperature of the fuel cell. It can be seen that as long as hydrogen and oxygen are fed to the fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.
In practice, a number of these unit fuel cells are normally stacked or xe2x80x98gangedxe2x80x99 together to form a fuel cell assembly. A number of individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. Fuel and oxidant are introduced through manifolds into respective chambers. An alternate style of fuel cell has been recently proposed (U.S. Pat. No. 5,783,324) which is a side-by-side configuration in which a number of individual cells are placed next to each other in a planar arrangement. This is an elegant solution to the problem of gas transport and mechanical hardware.
In most traditional fuel cell applications the fuel and oxidant supply streams are designed as flow-through systems. Flow-through systems add a parasitic load to the fuel cell output and thus reduces the net power that can be extracted from the fuel cell power source. In order to reduce the parasitic load, alternate configurations have been proposed in the prior art where the fuel stream or the oxidant stream or both are xe2x80x9cdead-endedxe2x80x9d. This dead-ended operation creates special problems. Two major problems with dead-ended fuel flow stream fuel cells are water removal and accumulation of impurities. These two problems lead to degradation of performance of fuel cells.
Design of fuel cells for portable applications need to be small and with air serving as the oxidant. Fuel cells for these applications are typically operated in a xe2x80x9cdead-endedxe2x80x9d fuel delivery system configuration with the cathode side open to air. A classical problem with these air breathing planar fuel cells is water management. Since the byproduct water is produced at the cathode, it evaporates away during normal operation. However, under heavy load, the evaporation rate lags the rate of formation and water tends to migrate back through the polymer electrolyte to the anode side. Some spots on a fuel cell are cooler than others, and the moisture condenses at these locations into liquid water, flooding the anode and impeding the flow of fuel to the anode.
Prior art recognizes problems with dead-ended operation such as impurity build-up and water accumulation at the anode. The accumulated impurities may poison the anode reaction sites. Inert contaminants would also result in loss of performance by lowering the fuel partial pressure. In the prior art (see, for example, U.S. Pat. Nos. 5,366,818 and 4,537,839), these issues are addressed by a brief controlled release of the fuel gas conducted at regular intervals. The purging operation involves controlled venting of a proportion (perhaps from 0.1 to 10%) of gaseous fuel or oxidant through a throttled opening. This purging action removes any accumulated impurities, water and fine particulates from the fuel cell and restores fuel cell performance. Many schemes have also been taught in the prior-art to control the length of, and intervals between, successive purges. One such scheme is to monitor data from the fuel cell power output to provide for the exhaust to be approximately proportional to the amount of hydrogen consumed by the cell.
Although purging can improve performance of dead-ended fuel cell systems by removing water and impurities, it wastes valuable fuel or oxidant and increases the parasitic loading on the system. It would, therefore, be an advancement in the art of fuel cell systems to have a dead-ended system that reduces or completely eliminates the need for purging.